A block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, which has a relatively high vinyl aromatic hydrocarbon content, has been widely used for injection molding or extrusion molding of a sheet, a film or the like by utilizing properties such as transparency or impact resistance.
Particularly, some proposals have been made with regard to a block copolymer and a styrenic polymer composition having a block copolymer blended. The proposed examples include a branched block copolymer of two linear copolymers before coupling, which comprises vinyl-substituted aromatic hydrocarbon blocks of a high molecular weight content and a low molecular weight content at a number average molecular ratio of from 3 to 7, and its production method (as illustrated in JP-A-53-000286) and a branched block copolymer comprising polymer blocks of at least 3 monomer units of vinyl aromatic hydrocarbons and its production method (as illustrated in JP-A-07-173232).
Further, there have been proposed a linear copolymer composition having a vinyl aromatic hydrocarbon block portion molecular weight distribution of from 2.3 to 4.5 or a branched block copolymer composition having a vinyl aromatic hydrocarbon block portion molecular weight distribution of from 2.8 to 3.5 which is prepared by blending (as illustrated in JP-A-52-078260), or a method of combining branched block copolymers having a vinyl aromatic hydrocarbon block portion molecular weight distribution outside the range of from 2.8 to 3.5 (as illustrated in JP-A-57-028150).
However, according to these methods, these block copolymers and compositions prepared by blending these block copolymers with various thermoplastic resins had a poor balance in respect of transparency and impact resistance, and a molded product prepared by injection molding under high shearing condition tends to easily generate anisotropic properties and the molded product thus prepared was unsatisfactory because strength became weak in one respect.